The invention concerns a control device for the position of a valve slide, with a d.c. voltage-operated set point transmitter, which produces a set point value proportional to the position of a control lever and the operating d.c. voltage, with an actual value transmitter operated with the same d.c. voltage as the set point transmitter, which produces an actual value signal proportional to the position of the valve slide and the operating d.c. voltage, the proportionality factors being equal to the dependence of the set point and actual value signals on the position of the valve slide or the position of the control lever, respectively, and on the operating d.c. voltage, and with a regulating device having a comparator, which regulating device makes the position of the valve slide follow the position of the control lever in dependence of a difference between the set point signal and the actual value signal detected by the comparator.
In a known, commercially available control device of this kind, the actual value transmitter and the set point transmitter are both made as potentiometers. Both potentiometers are connected to the same operating d.c. voltage and have the same transmission relation between inlet and outlet size. Therefore, they both produce a so-called xe2x80x9cratiometricxe2x80x9d signal, so that changes of the operating d.c. voltage have the influence on the control deviation or control difference, respectively, detected by the comparator, and thus on the outlet side regulating signal of the regulating device, without requiring a stabilisation of the operating d.c. voltage, regardless if changes of the operating d.c. voltage caused by interferences or by different choice of the operating voltage must be balanced. As the valve slide is connected with the outlet of the potentiometer converting its position to the actual value signal, the frequent displacements of the valve slide caused by control processes submit the potentiometer to a relatively fast wear, the potentiometer additionally being influenced by the working fluid, for example oil, of the slide valve, for example in connection with insufficient sealing of the valve slide in the valve housing, for example in that the working fluid penetrates between the outlet and the resistor of the potentiometer, thus changing the contact resistance between those.
The invention is based on the task of providing a control device as mentioned in the introduction, in which the actual value transmitter has a longer life, though still having a relatively simple embodiment.
According to the invention, this task is solved in that the actual value transmitter converts the position of the valve slide touchfree into the actual value signal.
In this embodiment, the actual value transmitter works substantially wearless, so that it has a longer life. A contact resistance, like in a potentiometer, is not available.
Preferably it is provided that the actual value transmitter has two coils wound in opposite directions and arranged coaxially after each other, the coils being supplied through an a.c. voltage generator, and a magnet core being displaceable by the valve slide relative to both coils in their magnet field. This gives a simple embodiment of the actual value transmitter.
The a.c. voltage generator can be a sine generator. Preferably, however, it is a triangle generator. However, a triangle generator, enabling a xe2x80x9cratiometricxe2x80x9d actual value signal in connection with the coils mentioned, is more simply designed.
This gives a more simple embodiment, when the two coils are connected in series, the a.c. voltage generator has a triangle pulse generator producing a one-sided triangle pulse and a transmission link with the transmission factor xe2x88x921, and the outlet of the triangle pulse generator is connected direct with one and via the transmission link with the other connection of the series connection of the two coils.
In particular, it may be provided that the triangle pulse generator has an integrator and a Schmitt-trigger, each having an operation amplifier, that the outlet of the integrator forms the outlet of the triangle pulse generator and is connected with the comparison inlet of the Schmitt-trigger, that the outlet of the Schmitt-trigger is connected with the inlet of the integrator, that a reference connection of the integrator and of the Schmitt-trigger lies on a reference level corresponding to half of the operating d.c. voltage, and that the outlet of the Schmitt-trigger lies on the operating d.c. voltage via a resistor. This gives a particularly simple embodiment of the triangle pulse generator.
For this purpose, the connections of both coils can be connected with the comparator of the regulating device via a phase sensitive rectifier, a lowpass filter and an amplifier, to rectify the a.c.-signal, picked up at the coils, representing not only the amount (size), but also the direction, of the displacement of the valve slide, into a d.c. voltage signal, which also displays the position of the valve slide according to amount and direction and still is ratiometric, so that in the comparator of the regulating device it can be compared with a set point signal also displaying a ratiometric d.c. voltage signal. In this connection, the amplifier can ensure that the proportionality factors or the transmission coefficients, respectively, of both transmitters are equal.
In a simple mannerxe2x80x94as usualxe2x80x94the set point transmitter can have a potentiometer, whose pick-up is connected with the comparator of the regulating device.